I haven’t bought Marigold seeds in at least 5 years because they are the easiest flower to collect seeds from. My main tips on collecting the seeds is: make sure they are dry (it hasn’t rained in a day or two) and store them in a paper bag. Oh, and the main tip is to LABEL the bag. This is more useful if you separate the seeds by color/variety.
Don’t know when the seeds are ready? Check out my photo below. The blue arrows are pointing to some that are ready. They should be easy to break off from the stem.
This is how I have been storing them the last few years. I always label it with the year, just in case I don’t use them all. I have had luck with them growing 2-3 years later.
Every Spring I throw the seeds by my tomato plants, in my flower pots around the house and in my wildflower bed. It’s fun to watch them grow and see how big they get.